During hot weather, adequate moisture is critically important to landscape plants. However, excessively wet soil combined with high temperatures can create stressful, and potentially destructive, conditions for bedding plants, perennials, vegetables, shrubs and even trees.
When the soil is saturated with water, pore spaces in the soil that normally hold air are filled with water. Since the roots of plants get the oxygen they need from the air in those spaces, the roots can literally drown when soils stay waterlogged for an extended period. A sick root system leads to a sick plant. Plants in this situation often lose vigor, look wilted, yellow, stunted or may even die.
These wet conditions also encourage fungus organisms in the soil to attack the roots or crown of a plant and cause rot. The crown is the area where the stem of a plant enters the soil. These disease organisms can cause dieback, severe damage or even kill plants. Once infection occurs, little can be done to help a plant. Plants with succulent stems (such as impatiens and begonias), those that like lower temperatures (such as geraniums and dianthus) and those that prefer drier, well-drained soils (such as Indian hawthorns) are particularly susceptible.
Gardeners can help to alleviate the situation. For one thing, adjust your irrigation systems that are on an automatic timer. It's not unusual to see sprinklers unnecessarily watering at homes or businesses the day after a heavy rain simply because the timer turned them on. Turn off the automatic timer if the weather is wet, and turn the system on only when drier conditions occur.
Always keep your beds well mulched to control weeds and maintain soil moisture. Under saturated conditions, however, the mulch should be pulled back from around plants or removed from beds entirely. This will allow evaporation to help the soil dry faster. Make sure you keep weeds under control while the mulch is off.
Shrubs and other plants affected by wet soils or root rot may look wilted even though the soil is moist. Leaf edges or entire branches may turn brown, and a bush may yellow and drop leaves. Shrubs showing these symptoms may ultimately be lost, but soil aeration in the root zone could help in some cases. Using a garden fork, drive the tines straight down into the soil about 8 inches and pull straight out in numerous places around the shrubs. Do not dig with the fork, but make as many holes as seems practical. This technique provides air to the roots and encourages the soil to dry faster. A metal rod or wooden dowel could also be used to make the holes.
Fungus diseases that attack the foliage of many plant are encouraged by rainy weather. Black spot on roses is prevalent now, even on fairly resistant varieties, and control is nearly impossible with constant rain. Cercospora leaf spot on crape myrtles is showing up now and causes the leaves to turn yellow or red and drop off. The disease is not fatal and the trees will recover without sprays, although flowering may be diminished. Lawn diseases, such as gray leaf spot, thrive in wet weather and are bound to be far more active as a result of our extraordinarily long rainy period. Leaf spots and fruit rots are likely to be more prevalent in vegetable gardens as well.
As the weather dries, a preventive spray of a broad spectrum fungicide, such as chlorothalonil (Daconil) or benomyl may be helpful on lawns, ornamentals and vegetables to prevent problems with leaf spots and fruit rots (Buckeye rot of tomatoes). Soil drenches with thiophanate methyl (Ferti-lome Halt) may improve the chances of ornamental plants affected by root rot disease.
Other pests such as snails and slugs thrive and reproduce rapidly during rainy weather. These pesky critters chew holes in the leaves and flowers of plants, and they are particularly fond of soft-leaved plants such as impatiens, begonias and hostas among many others. Try not to let their populations get out of control. If you have toads in your garden, that's great because they feed on slugs. There are numerous baits to help control snails and slugs. You can even place a bowl up to its rim in the ground and fill it half full of beer to attract and drown many snails and slugs.
Heavy rains over an extended period will leach available nutrients from the soil in the landscape. This is especially true of nitrogen and potassium, but not so much for phosphorous. If excess rain occurs during the summer growing season, you can evaluate your landscape plantings with this in mind. Look for foliage that is paler green than normal and growing slowly. Give your plants a chance to recover from the saturated soils, and fertilize if needed if they do not improve. Do not fertilize hardy trees, shrubs and lawns after August, even after heavy rains. Late fertilization can make them less winter hardy. Do consider fertilizing bedding plants and vegetable gardens, if needed.
If floodwaters have covered any part of your landscape, here are a few tips. Do not consume any vegetables that have been touched by the floodwater. There may be contaminants in the water. Once the floodwaters recede, promptly remove any debris or sediments covering the lawn. Hose off low-growing shrubs and bedding plants that may have been covered by floodwaters.
We have no choice but to work within the situations that nature presents us. Whether it is killing frosts, pest outbreaks, floods or high winds, we will always face challenges to our gardens. As always, we will clean up the mess and move forward to replant and repair the damage. Each new challenge teaches us important lessons and makes us more resilient gardeners.